As the art of naval warfare progresses, underwater sound detection equipment is required to detect fainter and fainter sounds at greater and greater distances. In order to detect underwater sounds the prior art deployed a plurality of hydrophones arranged in an array beneath the surface of the water as far removed as possible from foreign powers' ships. The array of hydrophones were deployed in a straight line beneath the surface of the water by stringing the hydrophones along a cable. The cable was towed by a surface ship, submarine, thruster or low flying helicopter.
In the past, each hydrophone of some arrays was hardwired to processing equipment that was at the end of the array cable. One of the disadvantages of the foregoing was that a separate wire had to be connected from each hydrophone to the processing equipment and when the array was large, i.e., 100 or more hydrophones, 100 or more wires had to be connected to the processing equipment. The many additional wires made the array heavy and it became difficult to keep the hydrophones in a straight line since the weight of the wires caused the array cable to bend. Thus, the number of hydrophones in the array had to be small because the processing equipment assumed that the hydrophones were in a straight line.
Another technique of the prior art for transmitting signals from hydrophones to processing equipment utilized multiplexing techniques. The multiplexing technique involved positioning information from each hydrophone on a different carrier frequency. Each carrier frequency was slightly displaced in frequency from the other carrier frequencies and before the information was received by the processing equipment this information was filtered or demodulated so that the output of each hydrophone would appear separately. One of the problems with the aforementioned system was that it did not have a high dynamic range, i.e., the ratio of the smallest amplitude signal that the system could process with a specified amount of noise to the largest amplitude signal the system could process with a specified amount of clipping or distortion expressed in db was relatively small. The same prior art system would not process faint noises that were produced by naval craft when they were far away from the sound detection system and loud noises that were produced by naval craft when they were close to the detection system. Thus, the prior art sound detection systems were limited in the amount of information that they could process. Hence, the same prior art sound detection system could not, for example, track a submarine as it moved from far away to close to the sound sensing devices, i.e., hydrophones.
Cross-talk or interference between two adjacent multiplexed channels due to mutual coupling between the frequency channels caused the sound detection systems to confuse the sensor of the information. For instance, the sound detection system might not be sure which hydrophone in the array received a particular signal, i.e., hydrophone number 48 or 49, etc. Prior art detection systems were able to overcome the foregoing cross-talk problem by increasing the frequency separation between adjacent channels. The foregoing solution caused the frequency bandwidth of the detection systems to increase. This meant that the number of hydrophones in the array must be decreased since there is a limit on how many different frequencies may be transmitted on a cable without the cable producing excessive transmission losses. One of the disadvantages of decreasing the number of hydrophones in the array is that the sensitivity of the array is reduced making it more difficult to locate objects in the water.